PROJECT SUMMARY/ABSTRACT Prostate cancer (PCa) is the second leading cause of cancer-related deaths in men in the United States and disproportionately affects African Americans (AA) compared to European Americans (EA). AAs are also at an increased risk of vitamin D deficiency since darker skin pigmentation inhibits cutaneous vitamin D synthesis. Vitamin D status is inversely associated with PCa aggressiveness and mortality and, therefore, vitamin D deficiency has been hypothesized as a biological contributor to the PCa disparity. Serum concentration of the pre-hormone, 25-hydroxyvitamin D (25D), is the clinical measure of vitamin D status, however, recent evidence suggests that the relationship between serum and tissue levels of vitamin D is more complex than previously thought. Despite having lower levels of 25D in the serum, AAs have higher levels of the active hormone, 1,25-dihydroxyvitamin D (1,25D), in the prostate tissue compared to EAs. AAs also have higher expression of the vitamin D receptor (VDR). Additionally, expression of the vitamin D metabolism enzyme CYP27B1, which converts 25D to 1,25D, positively correlates with percent West African ancestry. Levels of 25D in the prostate also inversely correlate with Megalin expression; Megalin is the extracellular receptor responsible for cellular uptake of 25D bound to its carrier protein, the vitamin D binding protein (DBP). The combination of these observations suggests an evolutionary conserved compensatory response in the prostate of African American men to ensure adequate levels of vitamin D in the prostate. The goal of the research proposed is to explore the hypothesized mechanism of prostatic vitamin D compensation. This will be accomplished by characterizing differences in vitamin D uptake and metabolism in primary prostate cells derived from AA and EA men. Additionally, the proposal will explore the role of Megalin in the prostate. This is of particular relevance since, in addition to 25D-DBP endocytosis, Megalin also binds sex-hormone binding globulin (SHBG) to facilitate androgen import. Since cancer of the prostate is androgen driven, this dual role of Megalin could have important implications for PCa initiation and progression. If Megalin does mediate internalization of both vitamin D and testosterone into the prostate it would provide an apparent mechanism of increased intra-prostatic androgen in the setting of vitamin D deficiency and could lead to increased risk of prostate cancer or more aggressive disease.